The multimarkdown option is not necessarily a good idea. The default
and recommended Markdown implementation in ikiwiki is Discount, which already
supports many of the same extensions as multimarkdown (in released versions of ikiwiki,
footnotes are a notable omission, but those will be enabled by default in the
next release).

If you want to use multimarkdown anyway, please check what the search path is
for the version of Perl that you are using to run ikiwiki. In particular, the
most reliable setup is likely to be one where you get Perl, Text::MultiMarkdown
and IkiWiki from the same vendor (for example Homebrew or pkgsrc) without
mixing those vendors.

If you obtained one of those packages (for example ikiwiki) by building it
yourself from source code, its author is unlikely to be able to help you
unless they happen to be a macOS user themselves, which most of the ikiwiki
contributors are not. If you are doing this, you should make sure you
understand how Perl finds libraries (for example this is described in
the perlrun man page). It might be necessary to add